1. Field of the Invention
The present invention relates to PCBs, and in particular relates to the formation of a bonding pad and a ring-shaped anti-etching layer of a PCB.
2. Description of the Related Art
A printed circuit board (PCB) is formed as follows. An electrical layout connecting elements of the PCB is drawn by circuit design, and electrical conductors corresponding to the layout are reproduced on an insulator by mechanical/chemical processes, surface treatments, and the likes. In the other word, the PCB is the substrate collocating the electrical elements, wherein the circuit formed by the electrical elements serves to deal the signals. The design quality of the PCB not only influences the electrical product confidence but also determines the product performance and competition. A circuit pattern is formed by processes such as printing, lithography, etching, plating, and the likes. Accurate lines are formed by the processes, to support electrical elements and connections therebetween. PCB technological trends strive for higher densities and multi-layered circuitry.
For multi-layered designs, the outer most metal patterned layer (usually a copper layer) of the component and solder side of a PCB, serves as a bonding pad to connect other electrical elements through the solder ball to the PCB, as shown in FIGS. 1A-1D. As shown in FIG. 1A, the core board 10 has a conductive through hole 12 to connect the laminar circuits 11 on the component side 10A and the solder side 10B. For a laminar circuit 11, the inner circuits 11A and the outer bonding pad 11B separated by the insulation layer 13 are connected by a blind via hole 14. As shown in FIG. 1B, a green paint 15 is formed to cover the bonding pad 11B, and an opening 16 is formed to expose a part of the bonding pad 11B. As shown in FIG. 1C, the metal protection layer 17 is formed on the exposed bonding pad 11B by a surface treatment process. As shown in FIG. 1D, the solder ball 18 is placed in the opening 16, and then solder is reflowed to form the solder bump.
However, devices manufactured by the method shown in FIGS. 1A-1D have some problems. First, the bottom of the green paint 15 adjacent to the opening 16 is easily influenced by etching or surface treatment, thereby being laminated from the bonding pad 11B and forming a void between the green paint 15 and the bonding pad 11B is formed. The void continuously enlarges during placing the solder ball and following solder reflow, such that the solder material will extend along the void to other regions, as shown in FIG. 1E and a local magnification in FIG. 1F. The described phenomenon causes shorts and lowers device confidence.
U.S. Pat. No. 7,213,329 discloses a method for forming a ring-shaped structure. In this patent, the seed layer is a collocated plated copper layer to form a dual-layered electrical connection pad, and the plating copper is etched through to part of the seed layer to form an opening. Thus, the contact area of the tin ball and the pad is increased, increasing sealing confidence. The described etching simultaneously removes part of the green paint, the plating copper layer, and the seed layer. Thereafter, the metal protection layer is formed on the seed layer by a surface treatment process, and the tin ball is placed by printing. However, the seed layer is often over etched, such that the opening is too deep to place the tin ball in it due to high aspect ratio. In addition, because the seed layer under the tin ball is too thin, the seed layer is easily combined with the tin ball to form an alloy layer. As such, the thin seed layer will be thinner, thereby degrading the product confidence.
Accordingly, a novel PCB structure is called desired to solve the described problems.